1. Technical Field
The technical field relates generally to vehicles providing for recapture of vehicle kinetic energy during braking and, more particularly, to control over blending of regenerative and non-regenerative braking on a vehicle to increase the proportion regenerative braking contributes to total braking, consistent with leaving directional control over the vehicle substantially unaffected.
2. Description Of The Problem
Many vehicles, including vehicles which provide for recapture of vehicle kinetic energy during braking (regenerative braking) provide such regenerative braking through the vehicle's drive wheels. In many cases this allows components of the vehicle drive train to function as a mechanism for energy recapture. For example, an internal combustion engine can be back driven from the drive wheels to operate as an air compressor. When operating as a compressor, the internal combustion engine draws intake air into its cylinders and compresses the air for delivery to a compressed air storage tank. Another possibility is a hydraulic hybrid which recovers kinetic energy by using a hydraulic motor connected to the drivetrain as a pump during braking and storing the fluid in a pressurized vessel. Alternatively, an electric traction motor can be back driven to operate as an electrical generator to charge a storage battery. Still other options include spinning up flywheels to store energy. Often these vehicles are not all wheel drive, meaning the vehicle drive train for such vehicles is connected to either the rear wheels, or the front wheels, but not both. As a consequence, often only the rear or front wheels of the vehicle are available for regenerative braking. On vehicles with less than all-wheel drive, the non-regenerative service/foundation brakes on each of the wheels frequently supplement vehicle braking, although this occurs primarily through the non-drive wheels.
Supplementing regenerative braking with non-regenerative service brake usage on a vehicle having less than all wheel drive serves a number of functions including providing predictable control over the vehicle, particularly if in a turn, by balancing the braking action among the wheels. Balanced braking usually aims at providing braking torque on each axle which is proportional to the weight carried by the axle. This provides control over vehicle yaw. However, the greater the non-regenerative braking torque provided, the smaller the quantity of energy recaptured for storage. Balanced braking can reduce energy recapture.
Under light or minimal braking, the difficulties posed by unbalanced braking are diminished. As a consequence, it is common to have a brake pedal “dead band” over a portion of the brake pedal travel from a fully undepressed position to a partially depressed position. In this dead band region, only regenerative braking is used. Service/foundation brakes are used to supplement regenerative braking as brake pedal travel increases. Vehicle weight changes and vehicle configuration changes can complicate application of this approach.
Regenerative braking is most commonly associated with hybrid-electric and electric vehicles. It is also found in vehicles providing for storing energy as hydraulic/pneumatic pressure and in a spinning flywheel, among other techniques. While it is anticipated that the present teachings will most often find application in vehicles having less than all-wheel drive and that typically regenerative braking will be provided through the drive wheels on such vehicles, it is not necessarily so limited.